Not my favourite film but the Three Musketeers would be a great Muppet movie.

D'Artagnan is a younger Muppet (Robin or Walter)

Kermit is Athos, Piggy is Lady de Winter, Gonzo is Aramis (except instead of wanting to join the priesthood while chasing women, he wants to join the circus while chasing chickens), Fozzie is Porthos

Pepe is the Man with the Black Moustache Sam is Cardinal Richelieu

Buckingham is Link Hogthrob